There have been a variety of centralizing mechanisms used to position tools within casing, drill pipe, tubing, and the like including drag springs, expandable and retractable linkages, cylindrical rubber posts extending laterally from the tool, metal fins, and the like.
In drill pipe or casing having internal upsets of reduced diameter, the centralizer must be able to flex or give slightly at the point of reduced diameter as the tool is being passed through the pipe, yet firmly and positively position the tool within the pipe when the tool is positioned for use. One example of such a tool as used within drill pipe is shown in U.S. Pat. No. 4,057,781, herein specifically incorporated by reference. Other such tools in use are disclosed in U.S. Pat. No. 3,494,186 and No. 3,410,136.
When such tools are installed in casing or drill pipe and used with fluid flowing through the pipe in high volume, the tool itself reduces the fluid flow cross-sectional area inside the pipe and the flow area is further reduced by the centralizing apparatus. It is usually necessary that the fluid flow area and fluid flow capacity through the pipe be maintained at or above certain specified minimum levels, both for the flow capacity and for providing reduced fluid drag on the tool itself.